1. Field of the Invention
The present disclosure relates to power line communications, and more particularly, to a technique of dealing with frequency nulls in a power line communication system.
2. Description of the Related Art
In a power line communication (PLC) system, a communication signal propagates over wires of an electrical power network. PLC signaling is typically performed by superimposing a high frequency signal, e.g., a frequency greater than 20 KHz, on top of a power line voltage. An electrical outlet in the power network can serve as both a source of electrical power and a port for the communication signal. Thus, a PLC transceiver plugged into the electrical outlet receives both electrical power and the communication signal via the electrical outlet. Note however, that some PLC devices do not necessarily receive power from the power line, or at least do not depend on the power from the power line. Therefore, in a PLC system, the power line is used for communication purposes, and in some cases, communication can be conducted either when power is present or is not present.
A null is a frequency at which the communication signal is attenuated to an undesirable level. Communication frequencies are subject to nulls caused by standing wave destructive interference. If a deep null occurs at a PLC system carrier frequency, communication may fail.
The frequency at which signal null occurs is a function of a complex impedance of the electrical power network. The complex impedance is, in turn, influenced by a presence of electrical devices or appliances coupled to the electrical network. It is also influenced by the numerous branch wiring circuits, which can each create reflections and affect impedances at varying frequencies.
The PLC system may be implemented in a single structure such as a residence or a commercial building, or over a complex of such structures. Thus the PLC system is not inherently limited to any particular geographic locality, size or topology, and it may include electrical wiring between structures.
The complex impedance is also influenced by operation of electrical devices or appliances coupled to the electrical network. For example, turning on an electrical light may cause a change in the complex impedance, and thus a change in the null. Consequently, the depth and/or frequency of the null can change.
Some PLC devices are very narrow band devices. U.S. Pat. No. 6,441,723 describes a device that uses a single frequency with a very narrow bandwidth. Consequently, if a null occurs at the single frequency, the device may be rendered inoperable.
Several techniques have been employed in an attempt to circumvent problems caused by nulls. A first technique is to use a PLC signal having a relatively low frequency, and thus a relatively long wavelength, so that the PLC signal is not affected by standing waves. Devices based on lower frequencies display a number of problems. First, to effectively communicate from one phase of the power line to another phase they typically require an installation of a “phase bridge” to couple the signals across the phases. This adds significantly to the cost of the installations and complexity since phase bridges are not always easily installed by a homeowner. Second, there is much more noise from appliances at lower frequencies, thus reducing the signal-to-noise ratio, which affects the quality of the signals. Third, capacitors in appliances, such as are common in televisions, have been designed to reduce interference but also can attenuate the desired communication signals significantly. A second technique is to have a plurality of frequencies available for PLC signals, and if communication is not satisfactory at a first frequency, then switch to a second frequency. The second technique requires PLC devices to be capable of operating at all of the plurality of frequencies, and additionally requires a control protocol so that the devices tune to an appropriate frequency. Consequently, as compared to a system that uses a single frequency, the second technique requires additional hardware and a relatively complex control protocol. A third technique employs spread spectrum technology such that a narrow null attenuates only a small portion of the PLC signal, yet other portions of the PLC signal survive to provide satisfactory communication. The third technique requires the use of a matched filter or other special receiving hardware to properly receive a spread spectrum signal that has been modified by a specific frequency null. Consequently, as compared to a system that uses a single frequency, the third technique requires additional receiver hardware and/or software.
It is desirable to maintain a communication system that utilizes a single frequency, because, as compared to an agile frequency system, the single frequency system is generally less expensive, of less complex design, and more reliable since it has fewer components. Accordingly, there is a need for an improved technique for dealing with nulls that interfere with the operation of single frequency, power line communication devices.